Elsevier

Applied Energy

Volume 213, 1 March 2018, Pages 293-305
Applied Energy

Experimental study on the performance of a vanadium redox flow battery with non-uniformly compressed carbon felt electrode

https://doi.org/10.1016/j.apenergy.2018.01.047Get rights and content

Highlights

  • Performance of VRFB with non-uniformly compressed electrode is investigated.

  • The Kozney-Carman constant for carbon fiber felt is modified.

  • Electrode intrusion ratio, local porosity and permeability are obtained.

  • Compression reduces electrode contact resistance and improves mass transfer.

  • Energy efficiency is improved to a maximum of 19.4% with compressed electrode.

Abstract

Optimal electrode compression can efficiently reduce electrode contact resistance and enhance species mass transfer so that the performance of vanadium redox flow battery (VRFB) is consequently improved. New designs of VRFB with a serpentine flow field on the current collector and compressed thin electrodes are investigated to increase its power density. In this study, the intrusion ratio, porosity, strain–stress, area specific resistance, hydrodynamic characteristics, and charge/discharge performance of VRFBs are comprehensively characterized under different compression ratios (CRs) which can be adjusted by changing the assembly force. Then, VRFBs using carbon felts with different CRs are tested by experiments, and the influence of electrode compression on VRFB cell performance is quantitatively evaluated. The in-homogeneous compression of carbon felt electrode in a VRFB with a flow field leads to a non-uniform porosity distribution of the electrodes under the channel, intrusion, and rib regions. The intrusion ratio, local average porosity, and permeability at different CRs are obtained. The Kozney–Carman constant of carbon fiber felt is modified by measuring the flow pressure drop through the electrode. The charge/discharge curves are acquired and the corresponding energy efficiencies are calculated under different CRs. It is shown that the charge/discharge time increases with the CR, and the energy efficiency can be improved to a maximum of 19.4% when the CR varies from 0.3% to 41.8%.

Introduction

The distributed renewable power sources, such as wind and solar power, have a mismatch problem between intermittent power supply and variable demands [1], [2], [3]. Under this circumstance, electrical energy storage (EES) technologies, including the lead-acid battery, Na/S battery, lithium ion battery, and redox flow battery can provide effective solutions for this problem [4]. Among different energy storage methods, the vanadium redox flow battery (VRFB) offers some advantages, including the flexible energy and power rating design, none cross-mixing of electrolyte, and long life cycle [5], [6], [7]. The VRFB has become a promising energy storage technology for distributed intermittent power generation and local grid electricity supply stabilization [8], [9].

The VRFB is an electrochemical system which stores energy in vanadium-based electrolyte prepared by dissolving vanadium ions in sulfuric acid solution. The electrolytes are circulated by pumps from the reservoirs to the electrochemical cell or cell stack. A typical cell comprises an anode, a cathode, and an ion exchange membrane separator which allows the diffusion of protons and prevents the direct cross-mixing of electrolyte solutions from the two reservoirs. Extensive studies have been conducted to optimize the structure design and the control strategy of VRFB to improve its performance. Most of these studies are numerical simulations using mesoscopic lattice Boltzmann method [10], [11], macroscopic finite element method [12], [13], and systematic models [14], [15], [16]. Besides, some studies focus on the modification of battery materials especially for electrode materials in order to ameliorate the VRFB performance. Currently, carbon fiber felt and carbon paper are the most commonly used electrode materials in a VRFB, and they play a very important role in determining the overall battery performance. Carbon felt is composed of carbon fibers which form a network structure in the felt and provide the path for the electrolyte transport. An optimal carbon felt electrode has the characteristics of the effective electrolyte transport, high conductivity, stable three-dimensional network structure, large surface area of active reaction sites, and good chemical/electrochemical stability. Many VRFB electrode materials are developed to enhance the kinetic reversibility and electrochemical activity of electrodes. Modifications of electrode material, which include acid, plasma, heat treatment, mild oxidation, gamma-ray irradiation, and deposition of conductive metals, are the main methods of improving VRFB energy efficiency [17], [18], [19]. Additionally, the composite electrode has been developed by using grapheme [20], [21], graphite [22], carbon, and metallic nanoparticles [23], [24], [25] to improve the battery performance. The exchange current density and electrochemical activity of vanadium ions on the electrode surface is improved by the modification of electrode materials due to the increased active surface area and the formation of oxygen functional groups [26].

During assembly, all the components of the battery are usually clamped together with appropriate compression in order to reduce the contact resistance between the current collector and the porous electrode material. Electrode compression during cell assembly may greatly affect the mechanical, morphological, electrical, and electrochemical properties of electrode materials so that the battery performance is altered. For a cell without flow fields, the electrode is uniformly compressed by a current collector. For a cell with flow fields, the use of channels can improve the flow distribution through the electrodes and reduce the flow resistance [27]. A reasonable flow field design can improve the mass transport with a low parasitic pumping loss. During assembly, the electrode is non-uniformly compressed owing to the rib-channel patterns of the flow field in the VRFB, which results in the intrusion of parts of the electrode into the flow channel. In fact, the intrinsic mechanism and influence factors of compression on the battery performance have not been fully investigated in the studies of VRFBs. Comparatively, the intrusion phenomenon and the effect of non-uniformly compressed electrode are widely discussed in fuel cell researches. Chi et al. [28] experimentally and theoretically investigated the effect of the non-uniform porosity of the gas diffusion layer (GDL) due to clamping force on the fuel cell performance. Tötzke et al. [29], [30] presented a synchrotron X-ray tomography study on GDL morphology under a flat and a flow-field compression punch. The results showed that the geometric properties of the GDL are changed significantly upon compression. Gaiselmann et al. [31] introduced a model which describes the three-dimensional microstructure of a uniformly compressed GDL with a flat stamp and a non-uniformly compressed GDL with a stamp with a flow-field profile. This work provided the morphological approach which is a good approximation of the real non-uniformly compressed GDLs for simulation studies.

For VRFBs, the effect of a uniformly compressed electrode on the performance of a VRFB without a flow field was experimentally explored by Park et al. [32]. The result indicated that the discharge time and maximum power increased with the increment of CR. However, the effect of the compressed electrode on the flow properties was not considered in their work. Latha and Jayanti [33] experimentally studied the effect of electrode compression on the permeability of the porous medium and the pressure drop in a VRFB with a serpentine flow field. The relation between the compression effect and the cell performance was not considered in this investigation. In the study of Chang et al. [34], the morphological variation of the electrode with channel and rib patterns was mentioned. The effects of felt compression on both the electrochemical performance and the pressure drop of a VRFB without flow field were investigated by Brown et al. [35]. The results showed that the increment of electrode compression contributes to a greater cycling stability. Oh et al. [36] performed a numerical study on the VRFB electrode compression behavior without flow field and reported that the higher energy efficiency is attained with increased electrode compression (20%) due to the significant improvement on the electronic conductivity of carbon felt. Nonetheless, the inhomogeneous compressed electrode was not quantitatively analyzed.

Previous studies of the electrode compression in a VRFB mainly focus on examining the charge/discharge performance of the VRFB. However, the relations between the cell characteristics and the different electrode compression ratios are not fully discussed or reported. Furthermore, the relevant researches do not consider the electrode intrusion into the channel for VRFBs with flow fields. Therefore, a comprehensive study of the influence of electrode deformation on the non-uniform properties of VRFB owing to the non-uniformly compressed electrodes with rib-channel patterns is necessary. In this study, the effects of electrode compression on the morphological, mechanical, fluid flow, electrical, and electrochemical properties of VRFBs are investigated in detail with different electrode CRs between 0% and 70%. Besides, the intrusion of the carbon felt into the flow field is also fully examined. The intrusion ratio, porosity, strain–stress, area–specific resistance (ASR), hydrodynamic characteristics, and charge/discharge performance are comprehensively characterized. During the charge/discharge test, the optimal CR for achieving the highest energy efficiency is obtained.

To address the issues mentioned above, detailed experimental studies are carried out in this work. The contents of the manuscript are arranged as follows. Firstly, the VRFB experimental system is described in Section 2.1, followed by a discussion on the methods used to determine the CR of carbon felt (Section 2.2), the mechanical and electrical characteristics (Section 2.3), and the pressure drop and permeability (Section 2.4). Then, the intrusion of the carbon felt is quantitatively evaluated and the intrusion ratios under different CRs are obtained in Section 3.1. Moreover, the variations of the compression strain and the electrical resistance respect to different CRs are discussed in Section 3.2. In Section 3.3, the permeability of the compressed electrode calculated using the Kozeny–Carman equation and the fitted Kozeny–Carman constant are compared. Finally, the overall effect of electrode compression on the flow properties and the charge/discharge performance of VRFBs are presented and discussed in Section 3.4.

Section snippets

Experimental setup of VRFB

A schematic diagram of a VRFB with a flow field is shown in Fig. 1. The flow field is slotted on one side of the current collector. The electrolyte firstly flows into the channel of the flow field and then transports into the porous electrode by convection and diffusion. During the charging/discharging process, the reactions which occur on the electrode surfaces can be expressed as follows:At the negative electrode:V3++e-DischargeChargeV2+At the positive electrode:VO2++H2O-e-DischargeChargeVO2

Morphology of inhomogeneous compressed electrode

The original carbon felt porosity (ε0) measured by water intrusion analysis is about 0.895. The pore sizes of the carbon felt are measured with micro-structure imaging (Fig. 8) using a Hitachi S-4800 field emission scanning electron microscope (SEM). As shown in the SEM image, the overlapped carbon fibers form a three-dimensional network structure. The carbon felt pore sizes are in the range from 10 μm to 270 μm, and the average fiber diameter (df) is estimated to be 11.94 μm. In Fig. 9, the

Conclusion

The inhomogeneous compression caused by the channel-rib patterns of the flow field results in a non-uniform porosity distribution within the channel region, the intrusion region, and the rib region. The main benefits of compressed electrode are the reduced electrode contact resistance and the improved species mass transfer. The electrode compression level has a large impact on the electrode material characteristics, which include the flow properties, physical parameters, electrical resistance,

Acknowledgement

The authors appreciate the support of the National Key Research and Development Program of China (No. 2017YFB0102703), the National Natural Science Foundation of China (No. 51536003), Shaanxi Province Youth Science & Technology New Star Plan (2016KJXX-56), and the 111 project (B16038).

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